Opportunities and Challenges for Renewable Power-to-X,ACS Energy Letters

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Opportunities and Challenges for Renewable Power-to-X
ACS Energy Letters ( IF 19.3 ) Pub Date : 2020-11-15 , DOI: 10.1021/acsenergylett.0c02249
Rahman Daiyan ₁ , Iain MacGill ₂ , Rose Amal ₁

Affiliation

Renewable power-to-X (P2X) is emerging as a viable platform for storing excess renewables for subsequent dispatch for end-use as well as providing a low capital-intensive decarbonization pathway to produce green fuel and chemicals.(1,2) In P2X, “excess” and underutilized solar and wind resources are used to power technologies that are capable of converting available abundant molecules such as water into hydrogen; CO₂ and water to methane, syngas, and oxyhydrocarbons; and air and water into hydrogen peroxide (H₂O₂) and ammonia. These energy carriers and chemical products provide significant versatility in renewable energy storage (to solve its intermittency), transport, and subsequent conversion to decarbonize the energy infrastructure. They also offer numerous advantages over alternative energy storage systems being trialed, such as battery and pumped hydro which are scale-, time-, and site-specific and cannot be used to transport energy over large geographical distances.(3) Adoption of P2X technology and products will further facilitate the integration of renewable power into other energy-consuming sectors that contribute greatly to the world economy (such as transportation, agriculture, and manufacturing), effectively displacing the requirement of fossil fuels.(4) This sector coupling benefit of P2X is of significance, because at present, renewable energy utilization is improving the CO₂ footprint of only the electricity sector (accounting for a third of global CO₂ emissions), whereas the remaining industries are going through a slower decarbonization route. At the core of renewable P2X technologies is electrolysis (Figure 1), which utilizes renewable electricity to split water/seawater into hydrogen and oxygen/chlorine; NO_x/nitrogen into ammonia; and CO₂ into CO, syngas, and formic acid. In addition, renewable hydrogen can be used in secondary conversion processes such as methanation, hydrogenation, and Fischer–Tropsch to generate a range of hydrocarbon products as well as in the Haber–Bosch process to generate ammonia. Notably, water electrolysis is seeing increased deployment around the world because of a strong policy push, with governments in Japan, South Korea, European Union (EU), and Australia rolling out separate national roadmaps and strategies that advocate renewable hydrogen as key for effective decarbonization.(5−7) Furthermore, the above-mentioned secondary CO₂ conversion technologies are also being demonstrated worldwide (e.g., synthetic methane production by Audi, Germany and synthetic methanol by Carbon Recycling International, Iceland).(8) Figure 1. Schematics of power-to-X infrastructure. Despite the technical and in some cases niche commercial maturity of key technologies and some early on-ground deployment, most renewable P2X products are still costlier when compared to their conventional fossil-fuel-based, production processes.(9) This price variation may arise because of the decentralized modes of P2X plants (most P2X plants are small-scale), whereas traditional chemical and energy industries take advantage of economies of scale and their ability to negotiate lower feedstock pricing. In addition, most P2X technologies suffer from relatively high capital costs (projected to decline as manufacturers ramp up their production capacity) and have large electricity requirement. In fact, there is a growing consensus that the cost-competitiveness of P2X is dictated by the availability of cheap and excess renewable electricity. Having said that, the evolving economics with these P2X technologies are promising, highlighting opportunity for commercial viability and scale-up.(10) Such economics are expected to improve further owing to advances arising from catalyst and system development for both electrolysis and secondary conversion reactor systems for different P2X pathways.(11) In addition to capital costs and electricity pricing, the costs of feedstock will also play a determining role in the price of P2X products. Particularly, for a number of secondary conversion processes, renewable power-to-hydrogen and its economics will play a critical role in their feasibility. In the following sections, we present the outlook and challenges of different P2X technologies and discuss pathways for cost reductions. Power-to-H₂. While renewable hydrogen pricing is at least two times more expensive than hydrogen generated from fossil fuels,(7) declining electrolyzer capital costs resulting from economy of scale and adoption of a new generation of cost-effective catalysts (either replacing commercial Pt/Ir catalysts or decreasing their loading) alongside declining electricity pricing is leading to competitive levelized costs across various jurisdictions.(12) While most work (research and demonstration level) with “renewable H₂” has focused on using clean water, in light of the previous drought experienced in Australia and North America and the lack of fresh water in Middle East and China, it is of utmost importance to explore the utilization of wastewater and seawater as feedstock. The direct utilization of seawater presents challenges, specifically for electrode and membrane stability (owing to a range of impurities present in seawater) and the formation of Cl₂ over O₂ for the anode reaction. Considerable research and funding is being directed to develop stable systems that can directly use seawater or through a combination of reverse osmosis water purification and electrolysis.(13) In contrast, the utilization of wastewater for electrolysis is more mature with a number of commercial systems capable of converting gray water to hydrogen.(14) It is expected that further understanding of the chemistry of these technologies may bring solutions to address the concerns of water issues in hydrogen production. Power-to-CO/Syngas/Formic Acid. Mimicking photosynthesis, CO₂ electrolysis provides an ambient approach to break CO₂ into CO, syngas, formic acid, methanol, and ethanol through the application of a small voltage. By powering these systems with cheap renewables and using cost-effective transition metals, CO₂ can be converted into CO, formic acid, and ethanol with a cost of $0.6, $0.79, and $1.46 per kg, respectively.(15) Note that the current market prices for CO, formic acid, and ethanol produced from fossil fuel sources are $0.06, $0.74, and $1 per kg, respectively.(15) Crucial to the cost-competitiveness of this route is low-cost CO₂ supply, as most reports with CO₂ electrolysis are with a pure stream (development of impurity-tolerant catalysts capable of directly converting flue gas is an urgent requirement). A low-cost supply can be attained by sourcing CO₂ from local emission-intensive industries using amine capture technologies and by locating P2X sites nearby. Alternatively, CO₂ can also be captured using direct air capture technology, directly contributing to reducing atmospheric CO₂ concentration and allowing flexibility in site location for P2X industries. While the current capture costs of direct air capture is high ($0.094–$0.232 per kg of CO₂), further R&D is expected to drive down the cost for direct air capture technology and make it competitive with amine capture technologies.(16) Power-to-NH₃. The global $80 billion ammonia fertilizer market is supplied by ammonia generated using the Haber–Bosch process that requires high pressures and temperature, as well as high-purity hydrogen (generated from fossil fuel) and nitrogen feed (from air liquefaction).(17) To decarbonize this hard-to-abate industry, a number of renewable power-to-ammonia routes are being actively investigated: (i) using renewable hydrogen for the Haber–Bosch process, (ii) conversion of pure nitrogen into ammonia using electrolysis (eNRR), (iii) plasma-driven conversion of air into ammonia, (iv) oxidation of pure air to nitrate and nitrite and subsequent reduction to ammonia, and (v) capture of NO_x from power plants in the form of nitrates and nitrites and subsequent reduction into ammonia (NORR).(18−20) Of these technologies, the adoption of renewable hydrogen (substituting hydrogen generated from fossil fuel) in Haber–Bosch is seeing increased adoption around the world, improving energy efficiency of Haber–Bosch from the current 15 kWh/kg_NH₃ to 8 kWh/kg_NH₃.(18) Moreover, of the direct conversion routes, NORR is emerging as a viable contender over eNRR owing to the availability of NO_x from power plants, mature capture technologies (such as wet scrubbing), and large ammonia production yield that can be conclusively demonstrated to come from nitrate reduction rather than impurities.(19,21) There remains considerable opportunity in electrocatalyst design and understanding of the underlying reaction mechanism for NORR alongside development of separation systems to remove NH₃. Power-to-Methane. CO₂ methanation technologies are widely deployed around the world to generate synthetic natural gas using the well-established Sabatier reaction, benefiting from immediate market demand, existing infrastructure for transportation and storage, and the lack of modification required for end-user applications. The economics of this exothermic reaction can be improved through utilization of transition metals (reaching ∼100% selectivity at ∼400 °C)(22) and further by meeting the temperature requirement using either renewable electricity-driven heaters and/or by harvesting infrared radiation of sunshine directly as a heat source. Given that we cannot completely steer away from fossil fuels in the short term owing to our reliance on chemicals and fuels (which are extracted from hydrocarbon), this renewable power-to-methane pathway is of significance as it provides an immediate route to close the carbon loop during this energy transition period.(23) Moreover, using renewable hydrogen to supplement this pathway would allow more incorporation of hydrogen within our energy mix while the infrastructure is being upgraded for the emerging hydrogen economy. Overall, the commercial feasibility of this P2X technology is highly dependent on the costs of renewable electricity and heat, as well as CO₂ feedstock, which may be sourced through the above-discussed amine-based CO₂ capture technology, which is seeing increased deployment worldwide.(24) It is predicted that through the optimization and system engineering, the cost of synthetic CH₄ may be reduced to ∼$20/GJ,(25) making the technology potentially competitive (with even conventional natural gas), and suitably sized renewable driven solar methanation units are simulated to be cost-effective and bring positive cash flow during operation. Power-to-Methanol. The conversion of CO₂ into methanol is another attractive direction because the high-energy-density liquid fuel can be readily stored, transported, and utilized for chemical manufacturing.(26) Specifically, the chemical is used as a raw material in the production of olefins, dimethyl ether, and formaldehyde, which serve the textile, packaging, and paint industries. The hydrocarbon is also used as a fuel substitute or additive through conventional combustion and through methanol fuel cells. There are now commercial technologies for renewable methanol production such as the ThyssenKrupp Uhde Methanol technology that utilizes renewable hydrogen generated from alkaline electrolyzer and waste CO₂ within a hydrogenation unit. On a commercial scale, pilot plants such as the George Olah Renewable methanol plant among others are already under operation.(8) Power-to-H₂O₂. The global $2.44 billion H₂O₂ market is currently satisfied through a well-established two-step anthraquinone route known as the Riedl–Pfleiderer process.(27) This energy-intensive process entails the requirement of fossil-fuel-sourced hydrogen, harmful organic solvents, and expensive Pd catalysts. Hence, to benefit from economies of scale, most H₂O₂ production facilities are large-scale and centralized to allow for cheaper levelized cost of peroxide production.(28) This, however, brings in additional safety issues and costs during transport and storage as concentrated H₂O₂ is hazardous. In contrast, renewable H₂O₂ generation using electrolysis will be beneficial as it would allow on-site decentralized production. Despite low yields, renewable H₂O₂ is able to satisfy most demands for water treatment and disinfection as these applications require only a dilute concentration. These systems can also play a vital role in a country’s first defense in decontamination of public places in response to viruses such as COVID-19.(29) Outlook. The potential economic benefits of renewable power-to-X is increasingly well-recognized by governments and industry, notably by companies engaged in the fossil fuel and its derivate industries who are at the forefront of global decarbonization efforts.(30,31) A testament to this is the sheer number of P2X demonstration plants being trialled worldwide: as of 2019, over 190 such plants are in operation to establish potential feasibility and scale-up of these various technologies.(8) It is expected that early opportunities for most P2X technologies would be small-scale individual/household applications (kW scale), before transitioning to a few to tens of MW for commercial and industry applications, and last in development of hundreds of MW to GW scale in hubs for national economy. The uptake of solar PV around the world featured such adoption in waves, beginning with rooftop solar, followed by the recent takeoff of utility-scale PV farms.(32) However, a number of these utility-scale PV projects are yet to be operational owing to grid overcrowding in Australia and Europe, highlighting opportunities to use this infrastructure to develop P2X units.(33,34) Hence, a key opportunity and challenge for the development of a P2X economy is through identifying and developing early niche applications with modest additional infrastructure requirements and promising commercial potential, which represent the different waves of transformation. Key factors in this include the scalability of technologies, early high-value product markets, manageable technology risk, and wider societal acceptance. Given current trends and market insights, it is conceivable that a potential P2X roadmap (Figure 2) may take the following shape: Figure 2. P2X deployment roadmap. The first wave of deployment will see increased usage on a household/individual scale such as decentralized hydrogen storage systems for offices and residential homes, fuel cell-powered material handling equipment, and on-site disinfectant and ammonia production. The second wave will encompass P2X products as feedstock and fuel for industry such as synthetic hydrocarbon and large-scale ammonia production from NO_x. The third wave of deployment will see integrated multiproduct P2X hub on a giga scale. (i) kW Scale. The first wave of transformation of P2X technology will entail the utilization of P2X product on a consumer level. This may include utilization of renewable hydrogen as an energy storage for domestic and office purposes (e.g., LAVO system)(35) or as a fuel in material handling equipment. At present, a number of retailers (Amazon and Walmart) are trialing hydrogen fuel cell for powering their electric forklifts. Another application of P2X technology at small scale can be on-site H₂O₂ generation as disinfectant for public places and transport and is becoming more significant in this pandemic age.(29) Decentralized ammonia production for greenhouses and small farms may also emerge as the first-wave technology, removing the need for fertilizer transport from manufacturing sites. (ii) MW Scale. MW size adoption of P2X technology will involve the generation of products for utilization as feedstock for industry. In this regard, P2X to hydrocarbon represents an ideal pathway as carbon capture from industrial and power plant sources benefits from economics of scale, and therefore, a large volume of CO₂ will be required to be converted into fuel and/or feedstock to make it commercially viable. Conversion of CO₂ via electrolysis to syngas, formic acid, and carbon monoxide is a MW scale application of P2X technology, and these products can either be retailed directly or used via secondary conversion technologies to generate a range of synthetic fuels, such as diesel, methane, methanol, and ethanol.(36) CO₂ can be also converted via hydrogenation to generate methanol.(26) Alternatively, wet-scrubbing can be applied to flue gas from power plants to capture NO_x in bulk quantities, which can be subsequently converted to generate ammonia, presenting an alternative to the 100-year-old Haber–Bosch process. Large-scale hydrogen bus fleets and transportation infrastructure may also emerge as a MW scale application of P2X technology, although the efficacy of fuel cell vehicles over battery electric vehicles remains to be seen. (iii) >100 MW to GW Scale. At this scale, P2X technology and products are expected to make a significant contribution to a nation’s energy mix. These economy-scale projects will entail production of bulk volume of renewable hydrogen and ammonia for export as an energy vector. For instance, the Pilbara region in Australia is being positioned to develop the 15 GW Asia Renewable Energy Hub, a major component of which would be converted into green hydrogen for export.(37) For domestic use, renewable-rich countries can blend renewable H₂ and synthetic natural gas (from CO₂ methanation) with natural gas to decarbonize the national grid and at the same time offset the requirement of additional fossil fuel.(7) In summary, we provide an outlook on the potential of renewable-energy-driven power-to-X as a sustainable pathway for chemical manufacturing and energy infrastructure transformation for renewable-rich regions around the world. We discuss strategies for improving cost-competitiveness of P2X processes and provide a roadmap for the different waves of adoption of P2X technology in the global economy. Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. The authors declare no competing financial interest. The work is supported by the Australian Research Council (ARC) Training Centre for Global Hydrogen Economy (IC200100023), ARC Research Hub on Integrated Energy Storage Solutions (IH180100020), and the Office of the NSW Chief Scientist & Engineer. We acknowledge Muhammad Haider Khan for assistance with the graphics. This article references 37 other publications.

更新日期：2020-12-11

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